A continuous casting mold flux is a powdery or granular auxiliary material used in steel making for covering the molten steel surface in a crystallizer of a conticaster. Due to high temperature of the molten steel, the mold flux comprises a solid layer and a liquid layer, wherein the molten layer is immediately adjacent to the molten steel, and the part of the mold flux above the molten layer remains in its original granular or powder form so as to achieve good insulation and thus preventing the molten steel surface from solidifying. On the other hand, due to the periodic vibration of the crystallizer, the molten layer flows continuously into a crevice between a copper plate of the crystallizer and an initial shell of the molten steel to lubricate the relative movement between the shell and the copper plate, such that good surface quality of a cast slab is guaranteed. In addition, the molten layer also has the functions of absorbing nonmetal inclusions ascending in the molten steel and purifying the molten steel. Generally, the mold flux film flowing into the crevice between the copper plate of the crystallizer and the shell is only 1-2 mm. One side of the film that is adjacent to the copper plate is in solid phase, while the other side adjacent to the shell is still in liquid phase. The liquid phase has a function of lubrication. The solid phase has good control over the capability of the copper plate of the crystallizer in cooling the shell, such that the cooling rate of the molten steel may be regulated and the effect in controlling heat transfer can be achieved. Hence, a mold flux is the last process technique for controlling the surface quality of a cast slab in steel making A mold flux with inappropriate properties may induce surface deficiencies such as flux inclusions, cracks, etc. in the cast slab. More seriously, the shell may even break and an accident of steel leakage may be incurred. Therefore, a mold flux is an important means for guaranteeing successful proceeding of a continuous casting process and surface quality of a cast slab.
Generally, a casting mold flux comprises mainly a binary system of CaO and SiO2, accompanied with fusion aids such as CaF2, Na2O, Li2O and the like to lower melting point and viscosity of the binary system of CaO and SiO2, as well as a small amount of such components as Al2O3, MgO, MnO, Fe2O3 and the like to obtain desirable metallurgical properties. Since the melting point of a mold flux is about 400° C. lower than the temperature of molten steel, an amount of carbonaceous material must be added to allow slow melting of the mold flux having a relatively low melting point on the surface of molten steel. The carbonaceous material, having a very high melting point, can stop agglomeration of liquid drops of the mold flux effectively, and thus retard melting of the mold flux. Among these components of the mold flux, the ratio of CaO to SiO2 (i.e. CaO/SiO2, referred to as basicity hereafter) and the amount of F may be regulated to achieve an effective control over the precipitation rate of cuspidate (3CaO.2SiO2.CaF2), in order to fulfill the purpose of regulating the crystallization behavior of the mold flux reasonably. Crystallization behavior is the most effective means for the mold flux to control heat transferring properties. Stronger crystallization behavior results in higher thermal resistance of the mold flux and lower heat transfer intensity. Fully vitrified mold flux has the minimum thermal resistance and the maximum heat transfer intensity. For low-carbon steel, ultralow-carbon steel and those types of steel having poor thermal conductivity (e.g. silicon steel, etc.), in order to improve the cooling of casting slabs, crystallization of the mold flux is undesirable. Hence the amount of F is generally low, specifically at about 3-5%. However, for peritectic steel and those types of steel containing crack-sensitive elements, if the cooling of molten steel in a crystallizer is uneven or too fast, the initial shell will break readily at weak locations under various stresses, resulting in longitudinal cracks. For these types of steel, the mold flux must have a very high crystallization behavior to effect slow cooling and inhibition of cracking. In these circumstances, the content of F in the mold flux is usually up to 8-10%. It can be seen that F contained in a mold flux not only acts to lower melting point and viscosity, but also plays an important role in improving crystallization. Thus, it is a very important component in a mold flux.
It is well known that F is a toxic element whose harm to human beings, animals and plants is at a level 20 times higher than the harm level of sulfur dioxide. Due to the high working temperature of the mold flux, which is generally at about 1500° C., a large quantity of environmentally harmful fluoride gases (including SiF4, HF, NaF, AlF3, etc.) are produced in the melting process. Fluorides, especially HF, in the air, are among the common air pollutants. Additionally, after exiting the crystallizer, the molten mold flux at high temperature contacts secondary cooling water sprayed on a cast slab at high speed, and they interact with each other to undergo the following reaction:2F—+H2O═O2-+2HF.
When HF dissolves in water, the fluoride ion concentration in the secondary cooling water and pH of the secondary cooling water are increased. As the secondary cooling water is recycled, the fluoride ions will be further enriched, and pH will be further increased. The increase of the fluoride ion concentration and pH of the secondary cooling water speeds up the corrosion of the continuous casting equipment greatly, leading to higher maintenance fee of the equipment, higher difficulty and neutralizer cost in treatment of the recycling water, and higher burden of sewage discharge.
In view of the above problems concerning a F containing flux, both domestic and foreign metallurgists devote themselves actively to development of environmentally friendly mold fluxes that are free of F. Currently, a relatively feasible solution is replacement of F with B2O3 which is batched reasonably with such components as Na2O, Li2O and the like to fulfil the purpose of regulating the melting property of the mold flux. See, for example, CN201010110275.2, CN200510065382, CN201110037710.8, JP2001205402, etc. However, the melting point of B2O3 is only on the order of 450° C., far lower than those of the other components of the mold flux. Hence, the softening temperature of the solid phase of the boron-containing mold flux is apparently lower. Consequently, the proportion of the solid phase in the flux film located in the crevice between the copper plate of the crystallizer and the shell is rather low, resulting in lowered thermal resistance of the flux film and rather high heat flow in the crystallizer. In addition, B2O3 contained in the mold flux tends to form a network structure, which inhibits crystallization. As a result, the solid phase has a vitreous structure. A vitreous solid phase has lower thermal resistance than a crystalline solid phase. Therefore, a boron-containing flux has lower thermal resistance than a traditional fluoride-containing flux. Once the excessively high heat flow exceeds the limit designed for a caster, not only the service life of the crystallizer will be affected, but the risk of sticking breakout will be increased. For ultralow carbon steel, the principal quality problem of a cast slab is the deficiency of flux inclusions due to embedding of molten mold flux in molten steel. For minimizing the possibility of embedding of the molten flux, one of the most effective measures is to promote separation of the mold flux by increasing the surface tension of the flux. However, B2O3 is a component capable of decreasing the surface tension of the molten flux. Hence, with regard to mold flux used for ultralow carbon steel, B2O3 is a component that must be controlled. The mold fluxes designed in patent application CN200810233072.5 and patent CN03117824.3 have an unduly high crystallization behavior and are suitable for crack-sensitive steel such as peritectic steel, etc. Patent applications JP2000158107 and JP2000169136 have proposed mold fluxes having high melting points and high viscosity, which are mainly used for billet continuous casting. In patent application JP2002096146, the MgO content is too high, such that Mg—Al spinel having a melting point higher than 2000° C. tends to form and worsen lubrication badly after the molten flux absorbs a certain amount of Al2O3.